1. Field of the Invention
Embodiments of the present invention relate to a circuit for detecting attacks by contacting an integrated circuit chip. This type of attack is generally called an “intrusion attack” and consists of applying conductive pins directly on areas of the chip to sample signals therefrom. In an integrated circuit, the active layers contain information processing circuits which may be sensitive, such as for example in credit card chips or access control chips in toll television applications.
2. Discussion of the Related Art
To protect circuits located in the active area from any fraudulent intrusion, it is known to use a screen covering the entire surface or an area thereof. This screen is generally formed of one or several metallizaton levels higher than the active area to be protected.
FIG. 1 shows, in a perspective view of the chip, a conventional example of an active area (ACT) covered with different metallization levels (M1 . . . Ms). A conductive path 2 delimited by terminals (V) and (W) is formed in one of these levels, for example, upper metallizabon level Ms. A vertical contact (via) enables connecting the conductive path formed in the higher metallizabon level (Ms) to other paths formed in the lower metallizaton levels.
The aim of these conductive paths is to detect a continuity interruption or any electric modification of their properties, for example, resistance and capacitance. These conductive paths run along the entire surface or only an area of the circuit to be protected, irregularly and randomly. If a “pirate” attempts to cross the metallization level containing the path, by introduction of one or several pins, a detection circuit is supposed to detect an interruption in the conductive path.
FIG. 2 very schematically illustrates in the form of blocks a conventional example of a circuit for detecting an attack against an integrated circuit chip.
In this example, several detection circuits (DET1, DET2, DET3) compare a random signal applied by a generator 1 (GEN RND) on a first terminal (V) of conductive path 2 with the signals present on several other terminals of the path. For example, any difference between the applied random signal and the signal present on output terminal W is detected and means an attack by contact with the chip, symbolized by alarm signal (AL) in FIG. 2. This detection may also be performed by conductive path sections, that is, at intermediary length levels (detectors DET1 and DE2 locating the section of the path where the attack has been conducted.
A disadvantage of such a solution is that is does not avoid for the conductive path to be duplicated by an external section to simulate an electric continuity.